If you want to accurately visualize the current trend in iPad sales, then this post from Dr. Drang covers it:
There are myriad other ways to smooth a graph without switching to cumulative figures. One of the simplest is the moving average. In this technique, instead of plotting the raw data, you plot the average of a few data points in the neighborhood of each time value.
For many data sets, the best size of this neighborhood is not obvious. With Apple’s sales figures, though, I think it’s clear that the best choice is to average over four quarters: the quarter that you’re plotting and the three previous. This smooths over the seasonal jumpiness while not including so much past data as to ignore real trends.
As for why iPad sales have hit this slump, I really liked this explanation from one of Drang’s readers:
Ben Packard, in an email, suggests my reason for dismissing the durability explanation for the iPad sales decline is weak. The Mac, he says, has been around long enough for there to be substantial numbers of owners at every stage of ownership, but far more iPad users are still on their first iPad. Whatever the long term replacement cycle of iPads turns out to be, we’re still in the first one, too early for variations in when people bought their most recent iPad to have evened out.